1. Field of the Invention
The invention relates to devices and methods for monitoring erectile events, and more particularly, for using penile impedance values in monitoring and evaluating nocturnal and diurnal erectile events for distinguishing between and/or diagnosing organic and psychogenic erectile dysfunction.
2. Description of Related Art
a. Erectile Dysfunction
Male impotence is defined as the chronic inability to attain and/or maintain an erection of sufficient rigidity for sexual intercourse. This problem affects approximately 30 million American men, with increasing incidence in those of advanced age. Impotence is a source of great anxiety for many and is the subject of many thousands of visits to physicians and other medical professionals every year.
During a normal erection, neurochemical stimulation causes penile arterial inflow to increase in each cavemosal artery. The result is increased blood flow into the corpora cavernosa. The subtunical venous plexus is compressed against the tunica albuginea, and venous outflow is reduced to trap blood in the corpora cavernosa. This combination of increased inflow and decreased outflow results in vascular congestion of the penis, tumescence, and rigidity sufficient for sexual intercourse.
It is believed that abnormal reduction of blood flow through the cavernosal arteries and/or excess venous outflow, i.e. corporal venous leakage, are the primary organic causes of vasculogenic impotence. These abnormal blood-flow characteristics through the cavernosal arteries can be caused by a number of factors, for example, diabetes, atherosclerotic vascular disease, traumatic arterial occlusive disease, or defective veno-occlusive mechanisms.
Erectile dysfunction can also originate in the mind, involving mental or emotional processes, i.e. it can have a psychological rather than physiological origin. Psychogenic causes include depression, anger/tension, low self-esteem, and fear. Additionally, organic impotence also often acquires a psychogenic component over time. Whether a patient's impotence is of organic or purely psychogenic origin, of course, determines the course of treatment.
b. Nocturnal Tumescence Monitoring
It is generally believed that patients with purely psychogenic impotence achieve normal erections nocturnally. Patients with organic impotence, on the other hand, generally suffer impaired erectile performance both nocturnally and while awake. Thus, measurement of nocturnal penile tumescence is a known technique for distinguishing between impotence of psychogenic and organic origin. Various devices and methods for performing nocturnal measurements have been developed.
One such method and device is disclosed in U.S. Pat. No. 4,515,166 to Timm, which is incorporated herein by reference. The disclosed apparatus, believed to be marketed as the RIGISCAN PLUS rigidity assessment system by Imagyn Medical Technologies, Inc., includes a number of loop-like structures that are positioned around the circumference of the penis. Periodically while the patient sleeps, a torque motor and an associated sprocket drive assembly exert a calibration force on the loop-like structures. Displacement of a cable connected to the loops while the torque motor is repetitively activated provides an output function that correlates to the compressibility or rigidity of the penis. Following a series of measurements, two-dimensional or three-dimensional graphic outputs of time versus tumescence can be created.
Another system for measuring nocturnal penile tumescence includes one or more mercury strain-gauge transducers. A loop-shaped tubing is filled with mercury and positioned around the patient's penis. As penile circumference changes, electrical circuitry detects changes in resistance values of the strain gauges to provide a rough indication of tumescence.
Another, even-less-precise method of monitoring nocturnal penile tumescence involves placing a ring of postage-type stamps snugly around the penis shortly before sleep. Upon awakening, the patient inspects the ring to determine whether any of the stamps are separated from each other, that is, whether the stamp ring has been broken. Although providing limited data, this method has certain advantages over merely subjective evaluations of nocturnal erectile activity.
Various penile tumescence monitoring devices and methods are disclosed in U.S. Pat. Nos. 4,469,108, 4,766,909, 4,747,415, 4,928,706, 4,428,385, 4,848,361, and 4,911,176, which are incorporated herein by reference.
c. Disadvantages of Current Monitoring Devices
Prior art methods and devices, however, suffer a number of significant disadvantages. Devices that surround the penis with constricting loops and then subject the loops to periodic, motor-induced tensioning may be viewed as somewhat intrusive and uncomfortable. Additionally, electronic-type monitoring devices are often of significant size, mass or volume, again contributing to patient discomfort and potential sleeplessness. Such devices also make travel impractical, possibly requiring an overnight stay in a medical facility for nocturnal monitoring and therefore causing significant inconvenience and expense to the patient and potentially interfering with nocturnal erectile performance. They also generally have significant power requirements and often provide only an indication of circumference at specific, spaced time-points.
Purely manual devices, of course, also fail to provide sufficient data for thorough evaluation and/or analysis; indeed, many prior art manual devices are so-called "one-shot" devices that merely indicate whether an erection has occurred, without providing more information.
In view of these disadvantages, a need has arisen for a monitoring and/or evaluation system of reduced size and otherwise unobtrusive construction, that provides enhanced data and more thorough evaluation and diagnosis capabilities.
d. Plethysmography--Introduction
Plethysmography is a known method for measuring blood flow/volume in the human body, most typically in the thoracic cavity. Various types of plethysmography are known, including volume-displacement plethysmography, strain-gauge plethysmography, segmental plethysmography, photoelectric plethysmography, and impedance plethysmography. These and other plethysmography devices and methods are described, for example, in Woodcock, J. P., "Plethysmography," Biomedical Engineering, September 1974, pp. 406-409, which is incorporated herein by reference.
e. Impedance Plethysmography
With impedance plethysmography, a constant, alternating current is caused to flow between electrodes spaced along the thoracic cavity, and a voltage drop between the electrodes is determined and correlated to impedance, or more specifically, bioimpedance. Tissue bioimpedance changes with variations in blood flow through the cavity. Various theories attempt to explain why this is so. According to one such theory, for example, bioimpedance changes over the cardiac cycle are due to the changing number of ions in the cavity when arterial inflow exceeds the rate of venous drainage. Other theories suggest that differing blood velocities or pressures, vessel distensibility, and/or orientation of red blood corpuscles contribute to bioimpedance changes. Typically, ventricular ejection causes an increase in blood volume and velocity, and corresponding changes in bioimpedance.
Both two-electrode and four-electrode impedance plethysmography techniques are known, for example as disclosed in Kubicek, W. G., et al., "The Minnesota Impedance Cardiograph--Theory and Applications," Biomedical Engineering, September 1974, pp. 410-417, which is incorporated herein by reference.
With the four-electrode technique, four aluminumized mylar strip electrodes are placed about the thoracic cavity of a patient--two electrodes generally in the neck region, one electrode at about the mid-point of the chest, and one electrode generally below the rib cage of the patient. The two outer electrodes supply constant, sinusoidal alternating current of 4 mA rms at 100 kHz longitudinally through the thorax of the patient. The two inner electrodes measure the potential difference between them along the thorax. More specifically, the product of the sinusoidal alternating current multiplied by the thoracic impedance generates a voltage between the inner electrodes, which is picked up by a high-input impedance linear amplifier. Outputs from the associated instrumentation include Z, which is the total impedance between the inner electrodes, .DELTA.Z, which is the impedance change during the cardiac cycle, and dZ/dt, which is the first derivative of .DELTA.Z with respect to time. Generally speaking, Z is related to tissue volume and dZ/dt is proportional to stroke volume, corresponding to the bolus of blood injected per heartbeat.
According to two-electrode applications of impedance plethysmography, a constant current passes through the thoracic cavity via the two electrodes. Potential drop across the electrodes is measured, and impedance between the electrodes is calculated based on the known current. Two-electrode impedance plethysmography is disadvantageous in that current distribution near the electrodes is not known precisely, and thus neither is the precise volume through which the current flows. Also, the impedance of the electrodes is added into the measurement, thereby biasing any calculation made with the impedance value itself. In four-electrode impedance plethysmography, on the other hand, current tends to spread homogeneously through the tissue, enabling more exact measurement.
f. Correlation of Impedance to Volume
Various equations have been proposed to correlate absolute volume and/or stroke volume per heartbeat with a bioimpedance variable. For example, from Bernstein, Donald P., "A New Stroke Volume Equation for Thoracic Electrical Bioimpedance: Theory and Rationale," Critical Care Medicine, 1986, pp. 904-909, and/or "NCCOM 3 Cardiovascular Monitor Operator's Manual," BoMed.RTM. Medical Manufacturing Limited, 1984, both of which are incorporated herein by reference, the following equation is derivable to determine thoracic stroke volume: ##EQU1## where: SV.sub.b1 =bioimpedance stroke volume (cm.sup.3); L=thoracic length, value from a nomogram that adjusts for deviations from ideal body weight and/or proportion (cm);
VET.sub.bi =ventricular ejection time by bioimpedance (seconds); PA1 (dZ/dt).sub.max =maximum rate of impedance change (.OMEGA./second); and PA1 Z.sub.0 =baseline impedance (.OMEGA.). PA1 L=mean distance measured between two (inner) electrodes (cm); PA1 Z.sub.0 =the mean thoracic impedance between two (inner) electrodes in ohms; PA1 (dZ/dt).sub.min =minimum value of dZ/dt occurring during the cardiac cycle in ohms/second; and PA1 T=ventricular ejection time in seconds, as obtained from the dZ/dt waveform or from heart sounds. PA1 Z.sub.0 =mean impedance between the measuring electrodes, in ohms; PA1 .DELTA.Z=impedance change during each heart beat, in ohms; and PA1 .DELTA.V=volume change, in cm.sup.3.
The Kubicek article referenced above also proposes a relationship between ventricular stroke volume and thoracic impedance change, according to the following equation: ##EQU2## Where: SV=ventricular stroke volume (cc); .rho.=electrical resistivity of blood at 100 kHz, average value is 150 ohm-cm;
The above equations assume constant thoracic circumference and/or volume. In the penile environment, on the other hand, circumference and volume are subject to significant change.
Kubicek also presents a derived formula to measure limb volume change without the use of a resistivity constant: ##EQU3## where: C=average circumference of limb segment under measurement, in cm; L=distance between measuring electrodes, in cm;
Pulsatile limb blood flow then equals the product of .DELTA.V and heart rate. Kubicek indicates that the above formula can be used in cuff-inflation venous-occlusion impedance plethysmography, in which case Z.sub.0 is the initial impedance before venous occlusion and .DELTA.Z is total impedance change between inflation and release of the cuff. Limb blood flow in ml/min equals .DELTA.V/.DELTA.T.times.60, where .DELTA.T is the time between inflation and release of the cuff, in seconds.
The above equation assumes constant length, which of course is an invalid assumption in the penile environment during an erectile event. Additionally, venous occlusion by cuff inflation in the penile environment is generally impractical, especially in a nocturnal monitoring situation.
Other equations relating impedance to stroke volume, cardiac output or other volume indicators have been proposed and will be available to those of ordinary skill in the art. Additionally, the following articles provide additional background information and are incorporated herein by reference:
Appel, Paul L., et al. "Comparison of measurements of cardiac output by bioimpedance and thermodilution in severely ill surgical patients." Critical Care Medicine, Vol. 14, No. 11, pp. 933-935,1986;
Bennett, Alan H., M.D., "Arterial and Venous Hemodynamics in Male Impotence," Management of Male Impotence, Vol. 5, pp. 108-126, 1982;
Bernstein, Donald P., M.D. "Continuous noninvasive real-time monitoring of stroke volume and cardiac output by thoracic electrical bioimpedance," Critical Care Medicine, Vol. 14, No. 10, pp. 898-901, 1986;
Chen, K. K., et al., "Sonographic Measurement of Penile Erectile Volume," J. Clin. Ultrasound, Vol. 20, pp. 247-253, 1992;
de May, C., M.D., et al. "Noninvasive Assessment of Cardiac Performance by Impedance Cardiography: Disagreement Between Two Equations to Estimate Stroke Volume," Aviation, Space, and Environmental Medicine, January, 1988, pp. 57-62;
Kedia, K. R., "Vasculogenic Impotence: Diagnosis and Objective Evaluation Using Quantitative Segmental Pulse Volume Recorder," British Journal of Urology, Vol. 56. pp. 516-520, 1984;
Kubicek, W. G., et al., "The Minnesota Impedance Cardiograph--Theory and Applications," Biomed. Eng., Vol. 9, pg. 410, 1974;
Nelson, R. P., et al., "Determination of Erectile Penile Volume by Ultrasonography," The journal of Urology, Vol. 141, pp. 1123-1126, 1989;
Nyboer, J., Electrical Impedance Plethysmography Springfield, Charles C. Thomas, p. 7, 1970;
Nyboer, Jan, et al., "Quantitative Studies of Electrical Conductivity of the Peripheral Body Segments," Annals of Western Medicine and Surgery, Vol. 5, No. 1, pp. 11-20, January, 1951;
Preiser, J. C., et al. "Transthoracic electrical bioimpedance versus thermodilution technique for cardiac output measurement during mechanical ventilation," Intensive Care Medicine, Vol. 15, pp. 221-223, 1989;
Quail, A. W., et al., "Thoracic Resistivity for Stroke-Volume Calculation in Impedance Cardiography," J. Appln Physiol., Vol. 50, pp. 191-195, 1981;
Schmidt, H. S. et al., "Significance of Impaired Penile Tumescence and Associated Polysomnographic Abnormalities in the Impotent Patient," The Journal of Urology, Vol. 126, pp. 348-352, 1981;
Sexson, William R., M.D., et al., "Cardiothoracic variables measured by bioelectrical impedance in preterm and term neonates," Critical Care Medicine, Vol. 19, No. 8, pp. 1054-1059, 1991;
Wong, David H., M.D., et al. "Noninvasive Cardiac Output: Simultaneous Comparison of Two Different Methods with Thermodilution." Anesthesiology, Vol. 72, No. 5, pp. 784-792, 1990; and
Zuckier, Lionel S., et al. "A Nonimaging Scintillation Probe to Measure Penile Hemodynamics," Journal of Nuclear Medicine, December, 1995, Vol. 36, No. 12, pp. 2345-2351.
g. The Difficulties of Using Impedance Plethysmography in the Penile Environment
As mentioned above, impedance plethysmography has been used primarily in the thoracic environment, e.g. with two electrodes in the neck area and two electrodes at the mid and lower chest, as in the Kubicek article referenced above. The penile environment presents special problems, however, making it far more difficult to minimize artifactual effects than in the thoracic environment. (In fact, the difficulties in the penile environment have caused some to theorize that plethysmographic techniques in general can never be used in penile environments adequately to measure and evaluate erectile activity.)
Thoracic impedance plethysmography, for example, generally does not concern itself with significant size changes of the external thoracic cavity. The size of the penis significantly changes during an erectile event, however, by up to several times in circumference, length and/or volume. These increases in size cause increases in penile impedance, due e.g. to the changes in length, and therefore cause deviation from the thoracic model. Electrode spacing in the penis also is subject to change. Further, the skin of the penis moves even in the absence of an erectile event, making adequate electrical contact more of an issue. Additionally, patient movement and/or movement of the penis relative to the remainder of the patient's body also cause artifactual effects. These considerations are especially disadvantageous and confusing in nocturnal monitoring situations in which patient movement is not constantly observed.
Only the inventors have determined the usefulness of impedance values in monitoring and measuring of penile blood flow and other variables, and more specifically, for monitoring and evaluating diurnal or nocturnal erectile events. For the first time, the inventors have developed special apparatus and method features in accordance with this discovery, including features designed to greatly reduce the confusion and potentially false data interpretation associated with artifactual effects.